Sol–Gel Encapsulation of Thymol Blue in Presence of Some Surfactants

Abstract

Transparent monolithic discs were obtained in presence of different surfactants: sodium dodecyl sulfate (SDS), ethanediyl-1,2-bis (dimethyldodecylammonium bromide (Gemini 12-2-12) or alkyl hydroxyethyl dimethyl ammonium chloride (HY). The use of surfactants has modified the morphology and porosity of the silica host matrix for better sensing properties. The entrapment of TB into silica matrix has shifted pKa value to more basic and less acidic. The presence of Gemini 12-2-12 and HY surfactants has shifted pKa value to more acidic, whereas SDS has shifted pKa to more basic in comparison with that of entrapped TB/silica.

Keywords

Sol–gel method Thymol blue indicator Surfactants TB entrapped-silica

1 Introduction

Sol–gel process transparent porous matrix offers the possibility of doping various organic and inorganic molecules. Sol–gel act as optical transducer material for monitoring chemical and biochemical reaction initiated by sensing molecules entrapped in the porous silica matrix. Most of the reported sol–gel based sensors are concerned with pH measurements [1, 2, 3, 4, 5, 6] Therefore; these materials have potential applications in pharmaceutical, food and chemical industries [7, 8]. When these materials are doped with pH indicators, important parameters such as sensitivity, time response, reusability were examined. More recently different types of surfactants (cationic, anionic, nonionic and zwitterionic surfactants) have been widely employed to alter the host silica matrix for better sensing properties [5, 9]. The modification of silica structure is greatly dependent on the nature of surfactant present [5, 10]. These mesoporous materials are excellent not only for accommodating active dopants but also for sensing analytes to be easily diffused towards the sensing centers [4, 5, 10]. More recent research work were reported with respect new optical pH sensors [11, 12, 13]. The use of surfactants do affect the mesoporosity of host silica for accommodation the active dopants, with respect to the host silica surfactants are surely improve the mesoporosity and better accommodation sites within the silica network [10]. It also strongly affect the nature of interactions with the corresponding indicator [14]. In this research work, thymol blue indicator has been employed and encapsulated into mesoporous silica matrix. This indicator was selected, since it has two indicating pH working ranges acidic, neutral and basic and compared with previous reported analogous silica entrapped phenol red (PR) [14]. Two cationic surfactants Gemini 12-2-12 and HY and an anionic SDS surfactant were used to modify the mesoporosity of host silica matrix. The use of these surfactants was not only modified the host mesoporous silica for better sensing, but also to alter the pH of entrapped-TB silica system. The use of three different surfactants anionic SDS and cationic surfactants Gemini 12-2-12 and HY surfactants have showed different interactions between silica matrix, TB and surfactant. The changes of pKa values for entrapped TB versus pH were clearly evident and compatible with reported silica entrapped phenol red (PR) [14]. The role of using these surfactants (Gemini 12-2-12 and HY and SDS) has remarkable effects on the sensing properties, which are well and clearly explained in different schemes.

The absorption spectra of the free TB in solution (2 × 10−4 M) and TB-entrapped silica are depicted in Fig. 1. Two absorption bands were observed at 418 and 550 nm for the free TB in solution (Fig. 1a), whereas only one absorption band at 448 nm with red shift ca. 30 nm is observed for the TB-entrapped silica (Fig. 1b) [5, 16]. The absence of the peak around 550 nm for the silica entrapped TB is probably associated with a change of structure of thymol blue from partially acid form in solution towards the neutral form of TB in the silica solid matrix (Scheme 2).

In this work, different types of surfactants are applied for entrapment of TB indicator: SDS, Gemini 12-2-12, and HY. The optical properties for TB-entrapped silica in presence of different surfactants were monitored at different pH. It is found that the absorption spectra for TB-modified mesoporous silica in presence of surfactant is dependent upon the nature of surfactant used. This is probably associated with its interactions with silica. The nature of interaction is ascribed to the nature of head group of each surfactant. The insertion of the surfactant into the silica matrix is to obtain mesostructure with high porous silica material. The resulting mesostructure could be efficient to host the sensing molecules. The electronic spectra of TB-entrapped silica, TB-entrapped silica/SDS, TB-entrapped silica/Gemini 12-2-12 and TB-entrapped silica/HY surfactant are illustrated in Fig. 2a–d. The electronic spectrum of TB-entrapped silica/SDS shows two absorption bands at 448 and at 555 nm (Fig. 2b). The reason for this behavior is probably that SDS enhanced the protonation of TB by protons form silica silanols groups (Scheme 3).

The absorption spectrum of TB-entrapped silica/HY shows an absorption band at 448 nm with absence of the absorption band at 555 nm. This may indicate that HY surfactant has shifted the TB indicator into the neutral form. This behavior is due to strong electrostatic interaction between the neutral form of TB with HY surfactant within the host silica matrix (Fig. 2c) (Scheme 4).

The absorption spectrum of entrapped TB/Gemini 12-2-12 shows a different behavior with respect of BTB/Gemini 12-2-12, where only one form (neutral) is present instead of the presence of both forms (acid and base) in the case of BTB/Gemini [10]. The reason for this behavior is that TB is more acidic than BTB. It acts as retarding for protonation from silica silanols due to presence of Gemini 12-2-12 surfactant and shifts the equilibrium towards the formation of the neutral form of TB. This can be explained that Gemini 12-2-12 molecules are attracted to the neutral forms of TB and shifted the equilibrium to the neutral form species of TB. This results of a strong interaction between neutral form TB with both Gemini 12-2-12 surfactant and silica (Scheme 5).

3.1 Effect of pH

The electronic spectra of free TB show two indicating transitions at pH range 1–12 (Fig. 3). The first one occurs at pH 1–3 range, in which the acidic form (red color) was changed into the neutral form (orange color), the second indicating transition exists at pH 7–9 range, in which the neutral form (orange color) was changed into basic form (blue color). There are two equilibriums with two isosbestic points at 493 and 501 nm. The two pKa values 1.66 and 7.00 obtained are closed to literature values 1.7, 8.9 (Table 2) [17, 18]. At pH < 3, two absorption bands were appeared at 440 and 550 nm, whereas at pH 7–9, two absorption bands were observed at 440 and 600 nm.

When the TB-entrapped silica is treated with different pH 1–12, two isosbestic points are observed at 501 nm and 520 nm (Fig. 4). This is probably assigned for the presences of two equilibriums correspond to the presence of two transitions. The first transition occurs at pH 1–3 which is attributed to a change from acid form color (red color) to the neutral form (orange color).

The second transition is assigned at pH 10–12 which is due to a change in color from neutral form (orange color) to the base form (blue color). pKa values for the modified TB-entrapped silica are found 3.10 and 10.5 (Table 2) [15]. The shift of pKa1 to less acid and pKa2 to more base in comparison with that of free TB solution. The reason for this behavior is that TB became less acidic and more basic shift when it entrapped into silica (Scheme 6, structures II and III) [15].

The optical absorption spectra of TB-entrapped silica/SDS at different pH solutions pH 1–13 is described in Fig. 5. Two different indicating transitions are observed with two isosbestic points at 492 and 504 nm. The first transition at pH 1–3 is due to the change from red color (acidic form) to the orange color (neutral form). The second transition at pH 10–13 is due to change from orange color (neutral form) to the blue color (basic form). It is clearly observed that the change of TB from the acid form to the neutral form was fast upon change of pH from 1 to 3, but the change of TB from neutral form to the basic form upon change in pH from 10 to 13 is slow (Fig. 5). pKa values were found (2.44, 11.00, Table 2). This results of a shift of pKa1 to more acidic and pKa2 to more basic [19]. This may explain that the anionic SDS surfactant has a basic character and therefore it shifted the equilibrium towards slightly more acidic (structure II Scheme 7). In alkaline, the TB is shifted to more basic (structure III Scheme 7, base form) in presence of SDS.

The electronic spectra of TB-entrapped silica-Gemini (12-2-12) surfactant at different pH solutions showed two indicating transitions (Fig. 6). The first transition occurs at pH 1–3 is due to the change of the color from red color (acid form) to orange color (neutral form), while the second transition occurs at pH 8–12, where the orange color (neutral form) of TB is change to blue color (basic form). This may be ascribed to the presence of two equilibriums with two isosbestic points at 496 and 515 nm, pKa values were found 1.60 and 10.6 (Table 2). In the presence of Gemini (12-2-12), the hydrophobic non-charged TB indicator molecules has changed into hydrophilic TB− and TB2− species and strong electrostatic attraction between the anionic species of TB and the cationic Gemini (12-2-12) is expected. It has reported that in presence of Gemini (12-2-12), the acidity of silica silanols is enhanced [10] and pKa1 of the system is shift to more acidic in comparison with TB-entrapped silica in absence of Gemini (12-2-12) [14]. At low pH there was an electrostatic attraction between protonated TB, silica and cationic Gemini 12-2-12 (structure II Scheme 8). At base media, the anion forms of TB− are attracted towards the cationic Gemini (12-2-12) with no significant change in pKa2 value (structure III, Scheme 8).

The spectra of TB-entrapped silica in presence of HY surfactant showed two indicating transitions (Fig. 7). The first transition is observed at pH 1–5, due to the change from (acid form) to (neutral form). This suggests the presence of first equilibrium between acidic and neutral forms. The second transition is observed at pH range 4–10, due to change from (neutral form) to (basic form). This suggests the presence of second equilibrium between the neutral and basic forms. Two isosbestics were found at 494 and 509 nm respectively. pKa values are obtained from the titration curves 1.69 and 7.41 (Table 2) [14]. At acidic pH solution, there is an electrostatic attraction between the protonated TB with the anionic end and with the silica silanol from the other end (structure II, Scheme 9). It was found that the monitoring of TB sensor goes rapidly from basic to acid medium but the reverse direction goes slowly. At basic solution, the anion forms of TB− are attached to the cationic part of HY surfactant. The low pKa2 value 7.41 (Table 2) may explain that in basic solution, there was a weak interaction between HY and the deprotonated TB− (structure III, Scheme 9).

Table 2 shows the changes of pKa values of silica entrapped TB and compared with entrapped PR [14] in presence and absence of different surfactants: SDS, HY and Gemini 12-2-12. It is found that pKa1 is shifted to more acidic and pKa2 is shifted to less basic for silica entrapped PR in comparison with silica entrapped TB in absence of surfactants. This probably refers to the presence of isopropyl groups in TB as electron releasing groups. It is found that pKa1 is shifted to less acidic and pKa2 to more basic for entrapped PR in comparison with TB due to the presence of surfactants. The changes on pKa values is also observed for other silica immobilized indicators [20, 21, 22, 23].

3.2 Effect of Sodium Hydroxide

Figure 8 shows the effect of sodium hydroxide solution (1.0 M) on the TB-entrapped silica with and without surfactant. Firstly, in case of TB-entrapped silica, TB-entrapped silica/Gemini and TB-entrapped silica/HY, a change of color takes place from red (acidic form) to blue color (basic form) are shown in Fig. 8a, c, d. However, in case of TB-entrapped silica/SDS three forms are supposed to exist in equilibrium, where the change occurs form red color (acid form) to yellow color (neutral form) and finally to blue color (base form 111) Fig. 8b.

3.3 Reversibility of the TB

The TB-entrapped silica/SDS sensor is acted as reversible sensor as the pH changes forward and backward. The spectra obtained in the range from 250 to 700 nm. The spectra showed three forms with two equilibriums. Two isosbestic points were observed at 497 and 557 nm respectively, Fig. 9. It is found that the change from base form towards the acid form is more rapidly compared with the change form acid to base form.

UV/Vis spectra of TB-entrapped silica/SDS with HCl 1 M started from base to acid form

3.4 Polarized light microscope

Photographs were recorded for TB-entrapped silica indicator with and without surfactants (Fig. 10a–d) by using the polarized optical microscope. They showed the changes in surface morphology of the different materials. The alteration in the size and the shape of the particles in photographs is ascribed the use of surfactants. Both SDS and HY have significant smaller size particles in comparison with TB/silica. Gaussian distribution of particle size of TB-entrapped silica in different surfactants was reported in Table 3. Analyzing Gaussian particle size distribution, it is observed that the mean particle size of entrapped TB was within the range 1.25 ± 0.20 μm. However, the particle size has a smaller mean particle size 0.61 for both silica entrapped BT/HY and BT/SDS and 0.72 for TB/Gemini (12-2-12). The use of surfactants results in decreasing of the distribution range of particles size (Table 3).

4 Conclusion

Transparent monolithic discs of TB-entrapped silica were prepared in presence of different surfactants: sodium dodecyl sulfate (SDS) or ethanediyl-1,2-bis (dimethyldodecylammonium bromide (Gemini (12-2-12)) and alkyl hydroxyethyl dimethyl ammonium chloride (HY). The entrapment of TB into silica matrix results into a shift of TB to less acidic and to more basic. The presence of surfactants have major role on the entrapped-TB. Gemini 12-2-12 has shifted TB to more acidic and almost no significant shift in base solution. HY has shifted TB to more acidic and less basic; therefore, it resembles the behavior of TB in solution without surfactant. SDS surfactant has minor changes to more acidic and to more basic. It is obvious that the new TB-modified silica systems have widened the pH working range in presence of surfactants and it can used as pH sensors in particular at low and high pH values.

Notes

Acknowledgements

The authors would like to thank the Chemistry Department at Al-Azhar University of Gaza for its generous support.